Acoustic measurement systems exist in several variants and may be used in many different areas, for example in measuring level or volume in tanks, containers or similar, in measuring distance, in measuring of flow, in medical diagnostics, such as ultrasound examination, in position determination etc.
An example is an echo type acoustic system for liquid level measurement. In such a system an acoustic transducer is typically provided at the highest point in a container which contains the liquid, the level or volume of which is to be measured. The acoustic transducer is fed from a transmitter with a first electric signal. In response to this first signal the transducer generates an acoustic pulse, typically in the form of an oscillating wave, which is transmitted downwards towards the surface of the liquid. After reflection against the surface the pulse is again picked up by the transducer which in response thereof generates a second electric signal which is fed to a receiver. The time interval between the first and the second electric signal, i.e. the transit time of the acoustic pulse, is determined and the distance from the transducer to the surface of the liquid can be calculated with a knowledge of the propagation velocity of the acoustic pulse in the medium in question.
Obviously in connection with such a transit time measurement it is important to be able to make an accurate determination of the time of reception of the reflected pulse or echo.
US 2007/0186624 discloses an acoustic method for measuring a signal propagation time in a medical liquid, where an oscillator-like received signal is sampled during its first half-period and checked with the help of a selection criterion based on the area enclosed between the resting level and the received signal during the half-period. When the result of this check is positive an intersection between the received signal and the resting level is determined with the help of which the signal transit time is calculated.
However, amplification or attenuation of the received signal typically changes as a function of temperature of the fluid in which acoustic signal propagates. This may cause erroneous measurements in applications where the temperature is not stable.
In an effort to reduce measurement errors caused by temperature changes, U.S. Pat. No. 6,226,598 discloses a method where an ideal characteristic first period is defined, which is characterized by an ideal amplitude ratio between the amplitudes of the two lobes of the ideal characteristic first period. Then, for each period of a received sound signal, the amplitudes of the two lobes of the period under examination are determined, and a ratio of the amplitudes is compared to the ideal amplitude ratio. If the result of the comparison is greater than a threshold value, the period under consideration is considered as being noise, whereas if the result of the comparison is less than the threshold value, the zero-crossing between the two lobes is considered to be the first zero-crossing of the received signal.
However, in some applications this method may be too computationally demanding.